Mixtures. 41 has brought his ladle under the mouth of the cupola ; the other rod has a flat end, with a lump of clay adhering. As soon as the ladle is full, he applies the clay to the mouth in order to stop the flow. To tap with safety, he should stand on one side of the trough and use his tapping rod obliquely, while on stopping again, he should cut off the stream from the top side. Mixtures of Iron.—This is another art which nothing but observation and practical experience can reduce to a nicety, different mixtures being used for the same purpose by different founders; indeed, the success of certain firms depends in a great measure on the mixtures used. Still, a good idea can be given of what is required for each purpose. The varieties of pig iron having been already stated, we will now consider each separately. No. i is the weakest but most fluid of all the pigs, and may be used by itself for ornamental castings on account of the ease with which it fills the corners of the mould, but it is usual to mix it with ' scrap J to increase its hardness, ultimate strength, and closeness of grain. No. 2 is finer in grain and stronger than No. i, and is used wherever some strength is required with great fluidity. No. 3 combines the greatest degree of strength consistent with fluidity, and is therefore most extensively used, and in great favour with founders. No. 4 is the strongest pig for foundry use (the remaining numbers, 5 and upward, being only required for conversion into wrought iron), and is therefore used for heavy castings requiring strength, such as girders, columns, bed plates, &c. For strong castings two-thirds of No. 4 may also be used with one-third of No. i. Scrap is the name given to the broken up parts of old castings, which of course may be divided into good and bad scrap. Some founders place great reliance on it, using nothing but scrap with an admixture of No. i, say two-thirds of scrap to one-third of No. i, while others prefer using an iron like No. 3, mixing with it only a little scrap to strengthen it, and so produce a harder, close-grained casting. It is also a good plan to mix iron from different blasts.